1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) signal transmission method and apparatus using a plurality of transmission antennas.
2. Description of the Related Art
Of OFDM signal transmission apparatuses, especially, a system which transmits different data simultaneously through a plurality of transmission antennas can transmit data at high transmission. On the other hand, the data error rate characteristic readily degrades. A method is known, in which on the transmitter side, a signal known to receiver, called a pilot symbol is superposed on one or a plurality of specific subcarriers to form pilot subcarriers. On the receiver side, channel equalization or frequency offset compensation is done for each subcarrier on the basis of the pilot subcarriers, thereby obtaining a decoded signal with an improved data error rate performance.
When a single known signal is transmitted from a plurality of transmission antennas by using pilot subcarriers of the same frequency, as described above, the transmission signals of the respective pilot subcarriers interfere with each other to form directional beams. In the IEEE802.11a standard, there are 52 subcarrier populated and four of them are assigned as pilot subcarrier. However, the directional beam of all the pilot subcarriers are directed in almost the same direction because the interval (about 4.4 MHz) between the pilot subcarriers is smaller than the carrier frequency (5 GHz). In this case, the null points at which the electric fields of the respective directional beams abruptly decrease are also directed in the same direction. For this reason, it is almost impossible to receive the pilot subcarrier in the direction of the null points, and the reception performance drastically degrades.
To cope with this problem, Jpn. Pat. Appln. KOKAI Publication No. 2003-304216 discloses a technique, in which a pilot subcarrier is transmitted from only one transmission antenna, and null signals in the frequency band of the pilot subcarrier are transmitted from the remaining transmission antennas. According to this technique, the problem of mutual interference between pilot subcarriers, which occurs when pilot subcarriers are transmitted from a plurality of transmission antennas, is avoided. Hence, any degradation in reception performance due to directional beam formation can be prevented.
In the method of transmitting a pilot subcarrier from a single transmission antenna, like Jpn. Pat. Appln. KOKAI Publication No. 2003-304216, the total transmission power of the pilot subcarrier is low as compared to a system which transmits pilot subcarriers from a plurality of transmission antennas. This degrades the reception performance of the receiver.
When the transmission power of the pilot subcarrier from the single transmission antenna is made higher than that of the data subcarrier from each transmission antenna, the total transmission power of the pilot subcarrier can be increased, and the reception performance improves. However, when the transmission power of the pilot subcarrier from the single transmission antenna is increased, a variation in transmission power occurs in the frequency band of the OFDM signal. This may result in composite triple beat (CTB) or increase the dynamic range of the transmission signal to make the specifications (especially input dynamic range) of a digital to analog (D/A) converter of the receiver strict.